In a mixing bowl, whisk together the flour, baking powder, cinnamon, and salt. Set the mixture aside. In a separate large mixing bowl, mash the sweet potato, leaving only a slight meaty texture. Whisk together the sweet potato and the eggs. Whisk in the warm water, then the sugar, then the oil followed by the vanilla extract. Stir in the dry ingredients until homogenous then fold in raisins and walnuts. Spoon the batter into the prepared bundt pan. Bake until a toothpick inserted into the center of the cake comes out clean, about 1 hour. Let the cake cool in the tin for 15 minutes and invert onto a serving platter or cake stand.
Meanwhile, make the glaze:
Melt the butter in a medium saucepan with the rum. Pull from the heat and whisk in the confectioners' sugar until smooth.
Pour the glaze over the cooled cake. Slice and serve.